1. Field of the Invention
The present invention relates to an illuminating window type bright frame finder. It is applicable to finders of, for example, a lens shutter camera and video camera.
2. Description of the Prior Art
Nowadays, in a lens shutter camera, it is predominant that a standard image size (24 mm.times.36 mm) and what is called a panoramic image size (13 mm.times.36 mm) are optionally interchangeable. In accordance with this, a finder is prevailingly used in which an image of a visual field is interchangeable with another. Further, it is greatly required that a parallax between a photographic lens and a finder, which is unavoidable in the lens shutter camera, is automatically compensated by changing field images of the finder. An illuminating window type bright frame finder with a liquid crystal frame is proposed which is capable of changing field images without a complex mechanism (see Japanese Patent Application Early Laid-open Publication No. Sho 57-173824, for example). The liquid crystal frame has a twist nematic type of liquid crystal layer. The liquid crystal layer is placed between transparent electrode patterns determined according to the shape and position of the frame to be changed. A pair of polarizing plates are disposed before and after the liquid crystal layer. Directions of polarization of the polarizing plates are perpendicular to each other.
Light from a subject side passes through one of the two polarizing plates and the liquid crystal layer and then reaches the other one of the polarizing plates. If voltage is not applied to the liquid crystal layer, light which has passed through one of the polarizing plates becomes linearly polarized light and then strikes the other polarizing plate without changing the state of linearly polarized light. Accordingly, the linearly polarized light cannot pass through the other polarizing plate. On the other hand, if voltage is applied thereto, the direction of polarization of linearly polarized light made by passing through one of them is deflected 90.degree. when the linearly polarized light passes through the liquid crystal layer. As a result, the direction of the light which has passed through the liquid crystal layer is made parallel with the direction of polarization determined by the other polarizing plate. Accordingly, the linearly polarized light can pass through the other polarizing plate.
In other words, if, according to the shape of the frame, voltage is applied to the liquid crystal layer, light which has passed through one of the polarizing plates can pass through the other one through the liquid crystal layer and therefore the shape and position of the frame can be discerned. Therefore, the changing of patterns of transparent electrodes to which voltage is applied leads to the changing of the shape and position of the frame.
However, in the liquid crystal frame, polarization characteristics of a pair of polarizing plates are used in order to transmit or intercept light, and for this reason, if the total quantity of light striking one of the polarizing plates is 1, the quantity of light emitted from the other one of the polarizing plates is 0.5. That is, the quantity of light passing through the liquid crystal frame is 50 percent of the quantity of light striking the same. In practice, the quantity of light passing through the liquid crystal frame is less than 50 percent because of, for example, the absorption of the light by the polarizing plates and liquid crystal layer, but this is ignored for the convenience of description. In addition, in the illuminating window type bright frame finder, light which has passed through the liquid crystal frame is gathered into an optical path of a finder system by means of a half mirror, and as a result, only part of the light which has passed therethrough enters an eyepiece. Accordingly, in the conventional illuminating window type bright frame finder, there is a problem in that a frame image within a visual field is dark and cannot be clearly viewed because of low efficiency in use of light.
In order to solve the problem, an arrangement has been proposed in which light is obtained from the upper side of a camera body (Magazine: Photographic Industry, Mar. 1994, infra page 62). However, it is not necessarily possible to always carry out sufficient lighting from the upper side thereof. For this reason, it is impossible to say that the problem mentioned above has been completely solved.
Further, under the present situation of the industrial field in which a lens shutter camera small in size is promoted, it is preferable to make a finder itself compact.
It is therefore a first object of the present invention to provide an illuminating window type bright frame finder capable of obtaining bright frame images and bright visual field images of the finder even if a liquid crystal frame is adopted. It is a second object of the present invention to provide a compact illuminating window type bright frame finder.